This invention relates to fibers that are flash-spun from fully halogenated hydrocarbon polymers and a solvent, and more particularly to flash-spun fully halogenated hydrocarbon polymers in which a substantial number of the polymer's halogen atoms are fluorine atoms.
The art of flash-spinning strands of plexifilamentary film-fibrils from polymer in a solution or a dispersion is known in the art. The term "plexifilamentary" means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean film thickness of less than about 4 microns and a median fibril width of less than about 25 microns. In plexifilamentary structures, the film-fibril elements are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form a continuous three-dimensional network.
U.S. Pat. No. 3,227,784 to Blades et al. (assigned to E. I. du Pont de Nemours & Company ("DuPont")) describes a process wherein a polymer in solution is forwarded continuously to a spin orifice at a temperature above the boiling point of the solvent, and at autogenous pressure or greater, and is flash-spun into a zone of lower temperature and substantially lower pressure to generate a strand of plexifilamentary material. U.S. Pat. No. 3,227,794 to Anderson et al. (assigned to DuPont) teaches that plexifilamentary film-fibrils are best obtained from solution when fiber-forming polymer is dissolved in a solvent at a temperature and at a pressure above which two liquid phases form, which pressure is generally known as the cloud point pressure at the given temperature. This solution is passed to a pressure let-down chamber, where the pressure decreases below the cloud point pressure for the solution thereby causing phase separation. The resulting two phase dispersion of a solvent-rich phase in a polymer-rich phase is discharged through a spinneret orifice to form the plexifilamentary strand.
U.S. Pat. No. 3,484,899 to Smith (assigned to DuPont) discloses an apparatus with a horizontally oriented spin orifice through which a plexifilamentary strand can be flash-spun. The polymer strand is conventionally directed against a rotating lobed deflector baffle to spread the strand into a more planar web structure that the baffle alternately directs to the left and right as the web descends to a moving collection belt. The fibrous sheet formed on the belt has plexifilamentary film-fibril networks oriented in an overlapping multi-directional configuration.
Many improvements to the basic flash-spinning process have been reported or patented over the years. Flash-spinning of olefin polymers to produce non-woven sheets is practiced commercially and is the subject of numerous patents including U.S. Pat. No. 3,851,023 to Brethauer et al (assigned to DuPont). Flash-spinning of olefin polymers to produce pulp-like products from polymer solutions is disclosed in U.S. Pat. No. 5,279,776 to Shah (assigned to DuPont). Flash-spinning of olefin polymers to produce microcellular and ultra-microcellular foam products from polymer solutions is disclosed in U.S. Pat. No. 3,227,664 to Blades et al. and 3,584,090 to Parrish (assigned to DuPont).
The commercial application for flash-spinning has been primarily directed to the manufacture of polyolefin plexifilaments, especially of polyethylene and polypropylene. However, experimental work directed to the flash-spinning of other polymers, has been reported. For example, U.S. Pat. No. 3,227,784 to Blades et al. describes the flash-spinning of a solution of a perfluoroethylene/perfluoropropylene (90:10) copolymer from a solution in p-bis(trifluoromethyl)benzene (Example 30). Applicants are not aware of commercial flash-spinning of such fluoropolymers. U.S. Pat. Nos. 5,328,946 and 5,364,929 disclose solutions of tetrafluoroethylene polymers at superautogenous pressure in perfluorinated cycloalkane solvents.
As used in this application, "hydrocarbon" refers to organic compounds consisting primarily of carbon and hydrogen; "halocarbon" refers to organic compounds comprised exclusively of carbon and halogens; and "oxyhalocarbon" refers to organic compounds comprised exclusively of carbon, oxygen and halogens.
Highly fluorinated polymer and copolymer films exhibit a variety of outstanding characteristics such as excellent resistance to acids, bases, and most organic liquids under normal temperature and pressure conditions; excellent dielectric properties; good tensile properties; good resistance to heat and weather; a very high melting point; and nonflammability. Highly fluorinated polymers and copolymer films are extensively used in high value applications such as insulation for high speed electrical transmission cables. Flash-spun plexifilaments of highly fluorinated halocarbon polymers and copolymers should find wide use in other high value applications such as, for example, hot gas filtration media, pump packings, gaskets, and protective apparel. However, fully halogenated polymers such as Teflon PTFE and Teflon PFA have very high melting temperatures (327.degree. C. and 305.degree. C., respectively). In addition, they are among the most inert known compounds. Consequently, fully halogenated polymers such as Teflon PTFE and Teflon PFA are very difficult to dissolve, even at high temperatures and pressures. Due to the extreme chemical inertness and intractability of fully halogenated polymers, it had not been possible to flash-spin such polymers.
There is a need for plexifilaments, microcellular foam fibers and microcellular foam sheets comprised of highly fluorinated polymers and copolymers that exhibit excellent heat and chemical resistance, good dielectric properties, and good non-stick characteristics. There also is a need for a process suitable for use in commercial flash-spinning of highly fluorinated hydrocarbon polymers using conventional spinning equipment under commercial temperature and pressure conditions.